The present invention relates to an image forming apparatus for allowing toner to fly in a direction from a toner carrier to a counter electrode so as to directly form an image on a sheet transported between the toner carrier and the counter electrode. The image forming apparatus is applicable to various devices such as a printing section of a digital copying machine and a facsimile device, a digital printer, and a plotter.
Conventionally, as an image forming apparatus for forming a visible image in accordance with an image signal on a recording medium such as paper, for example, the apparatus as disclosed in Japanese Unexamined Patent publication No. 211970/1992 (Tokukaihei 4-211970) has been available. The image forming apparatus (electrographic printer) as disclosed in this publication is provided with, as shown in FIG. 31, a development roller 102 and a background electrode 103. The development roller 102 is a carrier for carrying pigment particles 101 such as toner. The background electrode 103 is a counter electrode facing the development roller 102. To the background electrode 103 is applied a potential for drawing the pigment particles 101 in the direction towards the background electrode 103. towards the background electrode 103.
Between the development roller 102 and the background electrode 103 is provided an electrode matrix 104 (control electrode section) having a plurality of passages 105. The electrode matrix 104 is provided with electrodes, corresponding one to one to the passages 105, for controlling the electric field between the electrode matrix 104 and the development roller 102. The passage of the pigment particles 101 through each passage 105 is controlled by these electrodes corresponding to the passages 105.
Namely, in this type of image forming apparatus, flying of pigment particles 101 is controlled (allowed or prevented) by the electrode matrix 104 under a strong electric field generated between the development roller 102 and the background electrode 103. This allows the pigment particles 101 held on the development roller 102 to pass through predetermined passages 105 to reach a sheet 106 transported between the electrode matrix 104 and the background electrode 103, thus directly forming an image made of the pigment particles 101 on the sheet 106.
Therefore, the above image forming apparatus omits a developing medium, such as a photoreceptor for carrying an electrostatic image or toner image in accordance with an image signal, and a dielectric drum for temporarily carrying a toner image before transfer of the toner image formed on a photoreceptor onto a sheet takes place. This reduces the number of components and simplifies the structure of the apparatus, thus realizing a more compact and less expensive apparatus with ease. Also, because the transfer process from the developing medium to the sheet is omitted, the image is prevented from deterioration, and the reliability of the image forming apparatus is improved.
The following describes the problems associated with the above image forming apparatus. To begin with, two arbitrary electrodes of the electrodes formed on the electrode matrix 104, corresponding to the passages 105 are electrodes 104m and 104n. Here, for example as shown in FIG. 32, when an image signal is simultaneously applied to the electrodes 104m and 104n, for example, an ON potential of 150 V is applied to each of the electrodes 104m and 104n for a predetermined period in synchronization with the image signal, and then, for example, an OFF potential of xe2x88x92200 V is applied to the each of the electrodes 104m and 104n. 
Incidentally, at the moment when the ON potential or OFF potential is applied to a single electrode, that is, at the moment when the potential applied to a single electrode is switched to the ON potential or to the OFF potential, typically, a transient current of, for example, about 140 xcexcA flows through each electrode. Namely, as shown in FIG. 33, for example, in the case where the ON potential is simultaneously applied to the two electrodes 104m and 104n in accordance with the image signal, a total of about 280 xcexcA of transient current is supplied as a peak current at one instant from a power source.
While such a transient current is minute through one to several electrodes, it amounts to a huge value when the ON potential is simultaneously applied to several hundreds to several thousands of electrodes, as in the case of carrying out solid-black printing for example. More specifically, for example, supposing 2560 electrodes are provided on the electrode matrix 104, corresponding to the passages 105, when the ON potential is simultaneously applied to these electrodes, a transient current of 358.4 mA (140 xcexcAxc3x972560=358400 xcexcA=358.4 mA) is supplied from the power source at one instant.
Here, it is not generally preferable to adopt a power source having a large current capacity capable of supplying a transient current of, for example, 70 mA or more, as a power source for applying a high voltage such as the ON and OFF potentials to the matrix electrode 104. This is because when the current capacity is large, in the event of accident, such as a high voltage leak in which the high voltage applied to the electrode matrix 104 is leaked via sheet 106 as the sheet 106 is brought into contact with the electrode matrix 104 when printing, and malfunctioning of the apparatus, the output potential does not drop due to the fact that the current supplying ability of the power source is large, and as a result the high voltage flows into other circuits, causing troubles such as destruction of the circuits. Also, when other devices, such as a computer, are connected to the image forming apparatus, such devices may be destroyed as well. Further, in the worst case, if the user happens to be touching the image forming apparatus, the user may be electrified by the occurrence of such an event.
To prevent such an accident, a high-level insulation may be provided for the parts of the apparatus to which the high-voltage is applied or even for the entire apparatus. Yet, this is not preferable because of the high cost associated with this action.
Thus, the conventional image forming apparatus has the problem that when the number of electrodes on the electrode matrix 104 is large, a transient current exceeding an acceptable value is supplied from the power source, and as a result reliability of the apparatus is lowered.
Also, because a power source having a large current capacity is required, another problem is presented that the size and cost of the apparatus are increased. This problem especially becomes serious when the image forming apparatus is compliant with printing of large sheet 106 or has high resolution. Further, in a high-speed image forming apparatus, the number of times the apply potential is switched per unit time becomes larger, and this necessitates a power source capable of supplying even larger transient current, causing the foregoing problems as well.
The present invention offers a solution to the above-mentioned problems, and accordingly it is an object of the present invention to provide an image forming apparatus which does not require a power source having a large current capacity even when the number of control electrodes is large, and thus capable of reducing the size and cost of the apparatus and also improving reliability of the apparatus.
In order to solve the above-mentioned problems, an image forming apparatus in accordance with the present invention includes: a carrier for carrying developer particles; a counter electrode positioned so as to face the carrier; a control electrode section having a plurality of passage pores constituting a passage of developer particles flying from the carrier to the counter electrode, and a plurality of gate electrodes formed one to one around the plurality of passage pores; and potential applying means for applying a potential that is in accordance with an image signal to the plurality of gate electrodes, the image forming apparatus characterized in that the potential applying means applies a predetermined potential to each of the plurality of gate electrodes so that a sum of current flowing through each of the plurality of gate electrodes at a predetermined time by application of the predetermined potential is smaller than a sum of maximum value of the current flowing through each of the plurality of gate electrodes.
With this arrangement, the potential applying means applies a predetermined potential to the gate electrodes of the control electrode section, thus controlling flying of the developer particles held on the carrier towards the counter electrode by the potential of each gate electrode.
Here, when the potential applying means applies the predetermined potential to the gate electrodes, a transient current momentarily flows through each gate electrode when the potential is changed. If the timing of applying the predetermined potential is the same, the transient current is supplied from the potential applying means simultaneously. This means that when the number of gate electrodes is large, potential applying means capable of supplying a large transient current is required.
However, in the described arrangement, the potential applying means applies the predetermined potential to each gate electrode so that the sum of current flowing through each gate electrode at a predetermined time is smaller than the sum of maximum value of the current flowing through each gate electrode.
The potential applying means may realize this control, for example, by shifting the timing of applying the predetermined potential by a predetermined amount with respect to each gate electrode, or by grouping the plurality of gate electrodes, and while simultaneously applying the predetermined potential to the gate electrodes of a same group, applying the predetermined potential to the gate electrodes of different groups at a different timing. Here, it is preferable that the shifting of timing of set in accordance with a duration (transient response period) of the transient current.
This ensures that the amount of current supplied at a predetermined time by the potential applying means is significantly reduced compared with the conventional case where the potential is applied to the gate electrodes at the same timing.
As a result, it is not required to provide potential applying means having a particularly large current capacity even when the number of gate electrodes is increased.
Therefore, with the described arrangement, even when adopting a control electrode section having large numbers of gate electrodes, it is possible to use potential applying means having the current capacity which has been available conventionally, thus preventing an increase in size and cost of the potential applying means, preventing in turn an increase in size and cost of the apparatus.
Also, because the current capacity of the potential applying means is not particularly large, even when a high voltage leak, etc., is generated via the control electrode section, the output potential is reduced to some degree upon occurrence of such an event. Therefore, problems, such as destruction of other circuits and the apparatus, do not occur, thus improving reliability of the apparatus, and it is not required to take a high-level insulation measure.
Note that, as long as the transient current is flown as a result of applying the potential to the gate electrodes, the potential may be a potential for controlling flying of developer particles, such as a flying potential for allowing the developer particles to fly towards the counter electrode and a flying preventing potential for preventing flying of developer particles towards the counter electrode, or alternatively, a cleaning potential for cleaning the control electrode section.
In order to solve the above-mentioned problems, another image forming apparatus of the present invention includes: a plurality of carriers, each carrying developer particles of different color; a counter electrode positioned so as to face the plurality of carriers; a plurality of control electrode sections, each having a plurality of passage pores constituting a passage of developer particles flying from the plurality of carriers to the counter electrode, and a plurality of gate electrodes formed one to one around the plurality of passage pores; and potential applying means for applying a potential that is in accordance with an image signal to the plurality of gate electrodes, the image forming apparatus characterized in that the potential applying means applies a predetermined potential to each of the plurality of gate electrodes so that a sum of current flowing through each of the plurality of gate electrodes at a predetermined time by application of the predetermined potential is smaller than a sum of maximum value of the current flowing through each of the plurality of gate electrodes.
With this arrangement, the potential applying means applies a predetermined potential to the gate electrodes of each control electrode section, and as a result flying of developer particles of predetermined colors held on their respective carriers towards the counter electrode is controlled with respect to each carrier, thus forming a color image.
When the potential applying means applies the predetermined potential to the gate electrodes, a transient current is flown momentarily through each gate electrode when the potential is changed. Here, if the timing of applying the potential is the same, the transient current is simultaneously supplied from the potential applying means, and this means that when the number of gate electrodes is large, potential applying means capable of supplying a large transient current is required.
Also, in the above color image forming apparatus, for example, the number of control electrodes is four times that of the black-and-white image forming apparatus, and accordingly the sum of transient current supplied to each gate electrode by the potential applying means is also increased fourfold, requiring potential applying means having a current capacity four times larger than that of the black-and-white image forming apparatus.
However, with the above arrangement, the potential applying means applies the predetermined potential to the gate electrodes so that the sum of current flowing through each gate electrode at a predetermined time is smaller than the sum of maximum value of the current flowing through each gate electrode. The potential applying means realizes this control, for example, by shifting the timing of potential application by a predetermined amount when applying the predetermined potential to each gate electrode, or alternatively, by grouping the plurality of gate electrodes, and then by applying the potential to the gate electrodes of the same group at the same timing, while applying the potential to the gate electrodes of different groups at a different timing.
As a result, it is ensured that the amount of current supplied momentarily by the potential applying means is reduced compared with the conventional case where the potential is applied to the gate electrodes at the same timing. Thus, it is not required to provide potential applying means having a particularly large current capacity, even when the number of gate electrodes is increased.
Therefore, with the described arrangement, it is possible to use potential applying means having the same current capacity as that of the black-and-white image forming apparatus even when the control electrode sections having large numbers of gate electrodes are used, thus preventing an increase in size and cost of the potential applying means, and in turn size and cost of the apparatus.
Also, because the current capacity of the potential applying means is not particularly large, even when a high voltage leak, etc., is generated via the control electrode section, the output potential is reduced to some degree upon occurrence of such an event. Therefore, problems, such as destruction of other circuits and the apparatus, do not occur, thus improving reliability of the apparatus, and it is not required to take a high-level insulation measure with the described arrangement.
Note that, as long as the transient current is flown as a result of applying the potential to the gate electrodes, the potential may be a potential for controlling flying of developer particles, such as the flying potential for allowing the developer particles to fly towards the counter electrode and the flying preventing potential for preventing flying of developer particles towards the counter electrode, or alternatively, a cleaning potential for cleaning the control electrode section.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.